1. Field of the Invention
The present invention pertains to an image information detection system, such as a distance detection system or a focus detection system used in cameras, for example, that detects necessary information based on information taken from a pair of images that are formed on light receiving elements, as well as to optical equipment, such as cameras, binoculars and measuring instruments, in which the system is used.
2. Description of the Related Art
In a conventional camera having an autofocus function, the light from the object is led to a pair of line sensors to form two images, and focus adjustment for the photo-taking lens is carried out by detecting the object distance based on the difference in the location of the images. Specifically, using an optical system for distance measurement, the light from the object is led to a pair of line sensors each comprising photoelectric conversion elements and made to form an image, and correlation calculation is then performed in which the accumulated charges in the photoelectric conversion elements are compared between the two line sensors on a pixel-by-pixel basis, so that the distance between the two object images may be detected. The distance between the two object images is determined in accordance with the geometrical conditions of the distance measurement system, such as the distance between the distance measurement optical system and the line sensors, as well as the object distance. Conversely, when the distance between the two object images is obtained, the object distance is calculated in accordance with the geometrical conditions of the distance measurement system. Automatic focus adjustment relative to the object is carried out by controlling the motor that adjusts the focus position of the photo-taking lens in accordance with the object distance thus calculated.
In automatic focus adjustment in a single lens reflex camera, the light that enters the camera through the photo-taking lens is generally led to a pair of line sensors and a pair of object images are formed. Normally, the motor for adjusting the focus of the photo-taking lens is driven such that the distance between these two images coincides with the distance between two images that is achieved in in-focus state. In this case, automatic focus adjustment is performed directly without calculating the object distance, but the object distance may also be calculated where necessary based on the detected distance between the object images and the geometrical conditions of the distance measurement system.
In order to calculate the object distance using the method described above, it is required that the pair of images formed on the line sensors have adequate contrast. It is also preferred that the pair of images have approximately equal brightness.
For example, where the contrast is low, or in other words, where there is little difference in the accumulated charge among the photoelectric conversion elements of the line sensors, it is difficult to recognize the images, which in turn prevents accurate detection of the object distance or defocus amount.
Moreover, the user might block the optical path to one of the line sensors by mistake, or in a back-light photo-taking situation, extremely bright areas might occur in the image on one of the line sensors. In such cases, the reliability of correlation calculation is reduced, which prevents accurate distance measurement or accurate detection of the defocus amount.
As explained above, it is necessary to determine whether or not the contrast of the images on the line sensors is adequate, or whether or not there is a large difference in brightness between the pair of images. Therefore, conventionally, the contrast is calculated together with the correlation calculation, and determination regarding the brightness is carried out following the correlation calculation.
It is preferred that the images have a brightness equal to or higher than a certain level. If the images are uniformly dark, the reliability of the correlation calculation is reduced, which prevents accurate distance measurement or accurate detection of the defocus amount. As a result, the period of time in which photoelectric conversion and charge accumulation take place, or in other words, the integration time, is conventionally set at a prescribed period that would allow the images of an object having an average brightness to have adequate brightness.
However, this means that charge accumulation and reading are carried out at all times, and therefore it takes a long time to detect that the reliability of the correlation calculation is low or that the amount of light is insufficient. Where the reliability of the correlation calculation is low or the amount of light is insufficient, countermeasures may be taken, including providing a warning to the user and performing light emission from a distance measurement auxiliary light. However, where it takes a long time to detect that the reliability is low or that the amount of light is insufficient, as in the conventional model, implementation of these countermeasures will be delayed: photo-taking opportunities that the user seeks will therefore be missed because, for example, focusing of the camera cannot keep up with a moving object.